Saharawi women protest against Moroccan rule in Madrid. Such protests are forbidden and prevented by force in Western Sahara. Photograph: Andres Kudacki/AP

Western Sahara can only be described as a police state. I was there recently with the first British parliamentary delegation to the occupied territory and everywhere we went we were closely shadowed by undercover agents. Wherever we were driven by our Saharawi hosts, we were tailed by Moroccan police.

Most chilling of all was the heavy police intimidation of a peaceful Saharawi demonstration we witnessed in the capital, Laayoune, the day before we left.

The demonstration was the latest in a series of monthly protests called by human rights groups to demand the release of all Saharawi political prisoners being held in Moroccan jails, and an extension of the mandate of the UN monitoring body, Minurso, to include human rights.

Saharawi human rights groups had duly informed the Moroccan authorities of the protest in advance, but because all Saharawi organisations are banned, they were denied permission to hold the demonstration. The occupying forces in Western Sahara are not big on freedom of assembly.

Instead, those trying to make it to the protest found their way blocked by gangs of uniformed and plainclothes police and paramilitary auxiliaries to prevent the rally from taking place.

Wherever groups of Saharawi began to gather in surrounding streets, we saw police vans driven fast towards them and plainclothes officers jumping out to disperse people with baton charges.

The small numbers who did manage to make it through were immediately set upon. Next morning we were shown video footage of uniformed and plainclothes police surrounding protesters and roughly bundling them away.

We also met a number of those the police had assaulted, including one woman sporting bandages where she had been hit. Most shrugged off their injuries as an unavoidable hazard of activism under Moroccan occupation.

The delegation, which included the MPs Jeremy Corbyn and Mark Williams and the co-ordinator of the Western Sahara Campaign John Gurr, was in the territory to witness the human rights situation facing the Saharawi people after 39 years of Moroccan occupation.

We too had a brief taste of police harassment when the car in which we were driving was pulled over and impounded on the pretext that its papers were not in order.

While we were remonstrating with the crowd of plainclothes police who descended upon us, one reached into the car and snatched the camera with which we had been taking photos of the demonstration. We managed to retrieve it after making representations to the prefect of police, but all images of the rally had been wiped from its memory card.

In UN parlance, Western Sahara is officially a non-self-governing territory. This makes it Africa's last remaining colony, and no other country in the world recognises Morocco's sovereignty.

When Spanish colonial forces quit the area in 1975, the Moroccans moved in from the north and the Mauritanians from the south. Mauritania soon pulled its forces out again but Morocco stayed, launching successive waves of immigration into the territory that have turned the Saharawi people into a minority in their own land.

Any call for Saharawi independence is considered a crime against the integrity of the Moroccan state, as is showing the Saharawi flag in public. This did not stop several people proudly displaying it to us throughout our visit, as a sign of their refusal to bow to Moroccan military rule.

Morocco has pumped large amounts of money into Western Sahara to entice more settlers to move in and create the appearance of progress and normality. We were shown grandiose plans for public parks and municipal buildings in a development spree designed to turn Laayoune into a desert Disneyland.

In return, Morocco helps itself to the territory's natural resources, foremost among them phosphates, fish and the prospect of significant oil and mineral reserves.

As Europeans, we are also complicit in the dispossession of Saharawi resources. Last December our representatives in the European parliament signed us up to a new fisheries agreement with Morocco that allows European boats to fish in Saharawi waters in return for a healthy fee to the Moroccan authorities.

This was a major step backwards in the fight for justice in Western Sahara, not least because the parliament had previously voted against such a deal. Under international law Morocco has no right to trade away the resources of the Saharawi people.

We visited the fishing port close to Laayoune and saw the many boats moored there waiting to head out for the next catch. Almost all those employed in the industry are Moroccan settlers, with few job opportunities available for the Saharawi themselves.

All Saharawi activists we met were adamant that the only solution to their plight can come from the referendum on self-determination they were promised by the UN security council in 1991.

Until that promise is honoured, the struggle for Saharawi independence will continue.

• The delegation will hold a public meeting to report its findings in the House of Commons at 7pm on 25 February. It will also be addressed by the prominent Saharawi human rights activist Brahim Dahane. Full details can be found on the War on Want website.